<!DOCTYPE html>
<html>
<head>
	<meta charset="UTF-8">
	<title>Document</title>
	<style>
		body {
			height: 2000px;
		}

		.outer {
			width: 500px;
			height: 300px;
			background-color: red;
			position: absolute;
			top: 50%;
			left: 50%;
			margin: -150px 0 0 -250px;
		}

		.outer > .inner {
			width: 200px;
			height: 120px;
			background-color: green;

			top: 50%;
			left: 50%;
			margin-top: -60px;
			margin-left: -100px;
			position: absolute;
		}
	</style>
</head>
<body>

	<div class="outer">
		<div class="inner"></div>
	</div>
	
</body>
</html>